1. Technical Field of the Invention
This invention relates generally to crystal oscillator circuits, and in particular, to tuning of crystal oscillator circuits.
2. Description of Related Art
Local oscillator signals used in cellular telecommunications applications must be highly stable. Since crystals, such as quartz, have an extremely high Q, crystal oscillators are often utilized to provide the necessary frequency stability. Typically, quartz crystals are cut and mounted to vibrate best at a desired resonant frequency or an overtone (multiple) of the desired resonant frequency. When the crystal is vibrating, the crystal can be modeled as an RLC circuit that produces a rapidly changing reactance with frequency, with the RLC circuit providing positive feedback and gain at the resonant frequency, leading to sustained oscillations.
Although the frequency of a crystal oscillator tends to remain constant with a high degree of accuracy, aging, temperature and process variations of the crystal can lead to frequency shifts of approximately +/−30 ppm. However, some cellular standards require the frequency shift to be better than 0.01 ppm at 26 MHz. Therefore, crystal oscillators typically include some sort of frequency tuning mechanism to compensate for the inherent frequency shifts in crystal oscillators.
In many cellular applications, a digitally-controlled crystal oscillator (DCXO) is used to provide the necessary frequency tuning capabilities. DCXO's compensate for frequency errors using a combination of digital and analog circuitry. A DCXO “pulls” the crystal frequency to the desired value based on frequency measurement calculations. A digitally configurable interface can be used to programmatically add or subtract load capacitance to the oscillator circuit to change the resonance frequency of the crystal oscillator.
The load capacitance normally takes the form of a binary-weighted switched capacitor array. However, using conventional binary-weighted capacitor topology, it is difficult to obtain a 0.01 ppm frequency accuracy. Binary arrays produce code dependent glitches that are difficult to be eliminated. Since some frequency steppings are potentially much larger than 0.01 ppm, two different unit capacitors are typically required for fine frequency tuning. Using a larger unit capacitor in the fine tuning process can introduce significant glitches in the circuitry, which results in undesirable frequency variations in the oscillator output, thereby making it a challenge to arrive at monotonicity in frequency tuning. In addition, the regular layouts of traditional binary-weighted capacitor arrays have random mismatches, leading to non-monotonic frequency tuning.